The present invention relates to protective coatings for ceramic materials.
Advanced turbomachines use silicon (Si)-based ceramics such as silicon nitride, silicon carbide, and their composites for hot-section components. Due to the high temperature capability of Si-based ceramics, those ceramic turbomachines operate at higher temperatures with minimum cooling and higher engine performance. However, at operating temperatures above 1200° C., the Si-based ceramics can be adversely affected by oxidation and water vapor present in the flow stream. Such a hostile engine environment results in rapid recession of Si-based ceramics parts.
U.S. Pat. No. 6,159,553 discloses the use of tantalum oxide (Ta2O5) as coating material on silicon nitride parts. A tantalum oxide coating of 2 to 500 microns in thickness can effectively protect the surface of silicon nitride pans from oxidation and reacting with water vapor at high temperatures. However, there are some limitations on pure tantalum oxide coating on Si-based parts:
1. Ta2O5 undergoes a phase transformation from low temperature phase (beta-phase) to high temperature phase (alpha-phase) at about 1350° C., which may cause cracking in the coating due to the volume change occurred during the phase transformation.
2. Ta2O5 is susceptible to grain growth at temperatures above 1200° C. Pronounced grain growth results in large grain microstructure, which reduces the mechanical strength of the coating and induces high local residual stresses in the coating, and causes the coating to spall off.
3. Ta2O5 has a coefficient of thermal expansion (CTE) about 3×10−6 °C.−1, whereas silicon nitride has a CTE in the range of 3-4×10−6 °C.−1 and silicon carbide (SiC) has a CTE in the range of 4-5×10−6 °C−1. Since there is about 10 to 30% CTE mismatch between Ta2O5 and silicon nitride, and an even higher CTE mismatch between Ta2O5 and silicon carbide, residual stresses will develop in the Ta2O5 coating on Si-based ceramics. The residual stresses can limit the service life of the coating.
4. A pure Ta2O5 coating has low fracture toughness, which may adversely affect the mechanical integrity and the lifetime of the coating during service due to foreign object impact and particulate erosion events.
With those limitations, a Ta2O5 coating on Si-based ceramics would not provide adequate protection for turbine engine applications in which the maximum temperature goes above 1350° C., thousands of thermal cycles occur, and greater than five thousand (5000) hour coating lifetime is required. It would be highly desirable to significantly improve the Ta2O5 coating to meet those stringent demands for advanced ceramic turbine engine applications.